Hiro, a salaryman, picks up a succubus named Cubus who has collapsed from extreme hunger and ends up living with her in his home. Cubus' food source is semen and while Hiro initially struggles with this fact, he starts to see traces of the cat he rescued back at his parents’ house in the affectionate Cubus. As he keeps feeding her and she regains her strength, Cubus grows too big and round. Worried about her, Hiro suggests SEX as an exercise for dieting and so their peculiar life together begins...